Beijing -LRB- CNN -RRB- -- Demonstrators braved a heavy police presence and the threat of arrest by massing Thursday in the streets of Kunming , China , to protest the planned construction of a chemical plant , they said .

`` It was mostly a peaceful protest , '' said an activist who asked to be identified only by his family name , Young . `` We were singing the national anthem , shouting ` Get out , refinery ! ' together . ''

He added , `` We were scattered by the heavy police force in the area . I saw locals scuffling with police , people getting arrested and pulled away . ''

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The plant is a hot topic in the city , which is the capital of Yunnan province in southwestern China .

`` Every Kunming person cares about this issue , '' said another Kunming native , a 50-year-old man who said rows of police thwarted his efforts to reach the heart of the protest . `` The police kept blocking off the protest , block by block . ''

Local government officials did not respond to CNN phone calls for comment .

Photos posted on Chinese social media sites showed uniformed and riot police surrounding groups of demonstrators .

The five activists who spoke to CNN asked not to be named for fear of reprisals from Chinese authorities .

The oil refinery and chemical plant would be built in Anning city , about 50 kilometers -LRB- 31 miles -RRB- west of Kunming , according to Xinhua , China 's state news agency .

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Opponents fear the chemical plant would produce tons of paraxylene , a carcinogenic chemical identified by the acronym PX .

Several days after a May 4 protest , the Kunming mayor joined executives from the state China National Petroleum Corp. and the Yuntianhua Group for a joint news conference .

`` The government will call off the project if most of our citizens say no to it , '' said Mayor Li Wenrong , according to Xinhua .

The provincial general manager of China National Petroleum Corp. has said the refinery will not use the chemical .

`` The project has no PX facilities , nor will it produce PX products , '' Hu Jingke said , according to Xinhua .

Kunming residents expressed deep distrust of government officials and the state-owned enterprises behind the refinery project .

Several said authorities took draconian measures in the days running up to Thursday 's protest in an attempt to prevent demonstrators from taking to the streets .

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A 25-year-old Kunming native who asked to be identified as Claire told CNN that police detained and interrogated her for more than nine hours after she was caught distributing anti-refinery fliers on a city street Tuesday .

`` I understand we need oil refineries , '' Claire said in a phone interview . She said she opposed this one because `` it 's just so close to the city and the fact that the whole process was not transparent and we 're not allowed to ask questions . ''

Claire said police accused her of staging an `` illegal gathering . ''

As part of the interrogation , she said , police took her to the print shop where she had made copies of her fliers . Then they escorted her to her parents ' home , where they demanded that she erase files about the refinery from her computer . Finally , she said , they threatened her and banned her from attending Thursday 's protest .

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`` Basically , they 're saying if I have this on my record , I wo n't get any government job or state-owned enterprise job , '' Claire said .

Several other Kunming residents said students and employees of state-owned companies had been warned not to attend the demonstration .

`` Kunming is a beautiful city , where we have cleaner air than any other cities in this country , '' Young said .

`` We fear the refinery will destroy the city 's natural environment , and we demanded to see the environmental impact assessment report of this project . But we were rejected by the government who told us it 's a ` state secret . ' ''

An executive from Yuntianhua , which also has a major stake in the chemical plant , told journalists that an environmental impact assessment for the project had not been completed , Xinhua said .

In March , Chinese Premier Wen Jiabao promised that health safeguards would be improved and efforts would be made to control air pollution and make water supplies safer .

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`` We should adopt effective measures to prevent and control pollution and change the way we work and live , '' Wen said .

The emphasis on environmental and health issues comes as China 's leaders confront growing anger about choking pollution , contaminated food , and water that is unsafe to drink .

The statistics are staggering . China now burns 3.8 billion tons of coal each year , nearly as much as the rest of the world combined . In January , the smog over Beijing was so thick , it could be seen from space .

China 's CO2 emissions rose by 720 million tons in 2011 -- a 9.3 % increase . Pollution and dust generated in China have been found as far away as California .

Maintaining blockbuster growth has sometimes come at the expense of environmental protections .

The government has made explicit its intention to weigh environmental regulation against the cost to the economy . But analysts say the public outcry over pollution has tipped the scales .

China has already tried to boost the use of alternative sources of power , setting standards for solar energy installation and switching from coal to gas in some cities .

CNNMoney 's Charles Riley contributed to this report .

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Photos posted on social media show riot police surrounding demonstrators

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Protester : `` I saw locals scuffling with police , people getting arrested and pulled away ''

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`` Every Kunming person cares about this issue , '' another protester says

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Opponents fear the plant would produce tons of paraxylene , a carcinogenic chemical